Known fuel injectors, wherein fuel is supplied from an accumulator volume such as a diesel common rail, comprise a valve needle located for reciprocating movement within a bore of the fuel injector, under the control of a control valve, thereby to effect injection of fuel from one or more spray holes located in a tip of the nozzle body, into a combustion chamber.
Movement of the valve needle between open and closed positions is controlled by forces acting upon it resulting from a pressure difference between high pressure fuel in a barrel surrounding part of the valve needle, and fuel pressure in a control chamber surrounding a top end of the valve needle. The pressure in the control chamber volume, and therefore the forces acting upon the valve needle, are controlled by the control valve, and modulated by an inlet valve orifice (INO) and a restricted drain orifice (RDO), thereby influencing the motion of the valve needle, i.e. the rate of lift, damping, opening and closing velocities, and impact forces of the valve needle against upper and lower valve seats. However, the INO and the RDO are functions of fuel pressure within the accumulator volume, and therefore the degree of control they have on motion of the valve needle is restricted.
A known method of providing improved control over the movement of the valve needle is disclosed in European patent application no. EP0971118A (Isuzu Motors Limited), one embodiment of which comprises collar fitted to the valve needle, whereby the collar allows a limited, throttled fuel flow via a through hole located in the collar. However, as the collar is located in the barrel of the nozzle body, the effectiveness of the collar on improving needle motion control is sensitive to eccentricity in relation to the barrel bore. In particular, the location of the collar within the bore increases eccentricity of the collar due to a stack up of additional tolerances.